1. Field of the Invention
The present invention relates to direct-air cooling of board-mounted electronic components, and more particularly to an improved cooling device that can be mounted to individual electronic components to enhance the rate of heat transfer therefrom.
2. Description of the Related Art
As the power requirements of electronic components have increased, so has the need for efficient and reliable cooling. Failure adequately to conduct heat from power transistors and other highly dissipative components can result in destruction not only of the components themselves, but also of surrounding components as a consequence of heat buildup.
Manufacturers of apparatus that contain circuit boards typically employ two strategies to conduct heat away from vulnerable electronic parts. First, components that tend to generate the most heat are individually fitted with sinks that conduct heat away from the components. A heat sink, which as used herein refers generally to any device that conducts heat away from electronic components, usually contains a plurality of cooling fins that provide a large surface area (relative to the component to which it is attached). When introduced into an airflow, the fins promote convection of the heat away from the component. Thus, the second heat-reduction strategy involves directing a flow of air (ordinarily by means of an electric fan) at the components and providing ventilation ports through which the warmed air can exit.
Because the rate of heat transfer depends on, among other factors, the area of the heat sink and the velocity of the airflow, it might initially appear possible to maximize the efficiency of heat transfer simply by utilizing the largest heat sinks and air sources available. This approach is untenable from a purely practical perspective, since packaging considerations often severely restrict the space among components and between circuit boards (thereby limiting the allowable "footprint"--i.e., the horizontal space requirement --and height of a heat sink), and even modest improvements in airflow velocity tend to require considerable increases in power consumption.
Even without such constraints, large heat sinks can actually prove counterproductive in an environment crowded with electronic components. Assuming that air issues from a single source, any heat sink will block the flow and create downstream regions of thermal and velocity wake. Air in the wake moves slowly and randomly, retarding efficient convection. Although the wake has little effect on the transfer of heat from the wake-producing heat sink, it can significantly degrade the performance of downstream heat sinks disposed in the wake, since air in the wake is hotter and moves more slowly than the unimpeded airflow. The amount of wake created by a given heat sink depends on its profile. Accordingly, large heat sinks affect downstream components most severely, and it can prove difficult to design heat sinks that facilitate sufficient convection from one component without unacceptable effects on other components.
One proposed solution to this limiting tradeoff is a delta-wing design that produces longitudinal vortices in the direction of the airflow; see Lehmann & Huang, 171 Heat Transfer in Electronic Equipment 11 (ASME 1991). A longitudinal vortex will disrupt downstream wake, mixing the hot, slowly moving air in the center of the wake with cooler, high-velocity air outside the wake. This results in improved cooling of downstream components.
Unfortunately, the delta-wing design has drawbacks. It is mounted to the side of the component to be cooled, increasing the effective footprint. The device must also be placed directly in the path of the airflow. These orientation constraints impose design overhead on board architecture, since the layout must accommodate the vortex generators. Moreover, the delta-wing devices merely augment the performance of an existing heat sink by altering the airflow. They do not conduct heat away from electronic components. And finally, the relatively large profile of the delta-wing devices necessarily reduces the effective velocity of the airflow; although the worst effects of wake may be ameliorated, a reduction in velocity will nonetheless diminish the overall cooling rate otherwise obtainable with a given air source.